Physics AS electricity

Current and Charge

Current - a flow of electrons

each electron carries a charge of 1.6 x 10^-19.

Current is rate of flow of charge therfore-

Q=IT, charge = current x time

The simplest current is in a cathode ray tube, in a vacuumn, the cathode is heated releasing electrons towards the anode. It must be in a vacuumn to avoid collisions. Typically 5 x 10^14 pass a point in one second.

One coulomb is the charge supplied by a current of one amp in one second.

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current in a wire

Which metal ?

Copper for a connecting wire.

Copper or aluminium for a motor.

Tungsten for alightbulb filament.

Nickel- Crome alloy for a heating element.

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Current

Convential current flows from positive to negative, however, electrons really flow from negative to positive.

Ameters measure current-

Analogue ammeters rely on a small coil rotating in a magnetic field. Digital ammeters use a current sensor.

Ideally the reistance of an ammeter is zero so it does not affect the current it is measuring however in reality the resistance is very low.

Ammeters must be placed in series.

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Kirchoff's first law.

The sum of the current entering any junction is always equal to the sum of the current leaving the junction.

I = current, n = number density, A - cross sectional area of the wire,

v = mean drift velocity, e = elctronic charge (1.6 x 10^-19)

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Drift velocity in other materials

In conductors, there are large numbers of free elctrons so number density is high so current will be high.

In insulators there are low numbers of free elctrons so number density is low so current is low.

semiconductors have very low number density compared to conductors, however it can be increased by adding an impurity. The low number density means that drift velocity is much higher than in good conductors.

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Electro motive force (emf)

This is the energy transferred per unit charge when one other type of energy is converted into electrical energy.

Emf = energy transferred / charge

the unit is volt.

1 volt = 1 joule per coulomb

Typical values = 1.5 volts in a torch cell, 6 volts in a battery and 230 volts from the mains.

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Potential difference

This is the electrical energy transferred per unit charge when one other energy is converted into electrical energy.

Emf refers to electrical supply e.g. a battery, where as p.d refers to a component of a circuit. The unit is still a volt.

The voltmeter must be in parallel in a circuit and the resistance should be infinite but is in reality very high.